In the ocean, there are a multitude of underwater sensor nodes collecting data such as salinity and temperature. All these sensors require constant service in order to replace the battery to maintain operation. A primary method of charging these sensors is to remove them from the water, replace the battery, and re-deploy the system. However, this results in unwanted lapses in operation as well as high maintenance cost.
Another option is to charge the sensor in situ through wireless power transfer. Wireless power transfer systems are available in the consumer market for charging consumer electronics. They typically use a planar transmit coil to wirelessly transfer power to a planar receive coil, which then charges a battery in the electronics. While this configuration is manageable for charging consumer electronics, it is not functional for charging within the ocean environment, as there are several challenging aspects of charging in underwater environments.
One such challenging aspect is alignment of the coils, which is important for maximizing electromagnetic coupling. Ocean currents cause the coils to drift apart resulting in misalignment inefficiencies or greater standoff distances. Another challenging aspect is bio-fouling. Coils will heat up due to the electrical current passing through the coils. The heating will increase bio-fouling growth on the coils resulting in greater and greater standoff distances. The increased distances result in poor power transfer efficiency.
A further issue presented in the underwater environment is that ocean saltwater is a highly electrically conductive medium. This creates a number of issues including lowering possible frequencies of operation, higher coil radiation resistance, and eddy current losses. Each of these issues results in poor power transfer efficiency. The high electrical conductivity (4 S/m) of ocean saltwater limits the frequency of operations because of skin depth. Additionally, there is an increase in radiation resistance for coils in saltwater as the presence of ocean water increases the radiation resistance of the loop. For the coil in saltwater, the radiation resistance is very high, much higher than the other resistances that are present. Also, the coils suffer from eddy current losses in the ocean water. These eddy current losses are formed from small magnetic loops that run counter to the loop formed in the coils.
The obstacles discussed above result in poor charging efficiencies, which lead to longer charging times. A system and method are needed to address the above shortcomings and provide an efficient means for wireless power transfer in an underwater environment.